Phenomenological Introduction of Standoff Distance of Sonic Ring Impelling Entropy Waves and Aerodynamic Heating of Hypersonic Vehicles

Abstract

The fundamental understanding of the Sanal flow choking and/or sonic fluid throat effect (V.R.S.Kumar et al., Physics of Fluids, 34(4 & 10), 2022) in real-world fluid flow system sheds lights on the theoretical discovery of the standoff distance of the Sonic Ring/Jacket impelling entropy waves and aerodynamics heating on supersonic and hypersonic vehicles. Since all fluids in nature are viscous, all flying objects exhibit zero velocity over its surface. It implies that there will be several continuous sonic points away from the surface, which are around the supersonic/hypersonic vehicles. The line joining all these continuous sonic points defined herein as unique “Sonic Ring (2D case) and/or Sonic Jacket (3D case).” The distance of the sonic point from the surface of the supersonic/hypersonic vehicle is coined herein as the “Standoff Distance.” The Sonic Ring is a creation of series of sonic fluid throat effect because of streamline compression and flow choking, which occurs at a critical total-to-static pressure ratio. During the turning of the freestream flow over the vehicle due to the geometry effect or otherwise, streamline compression occurs due to gas stickiness because of the enhanced viscosity. The viscosity of the gas increases due to an increase in gas temperature. The freestream gas temperature increases due to the entropy waves originated from shock waves (oblique/normal shock). Oblique shock will occur when the hypersonic/supersonic flows encounter a corner that effectively turns the flow into itself and compresses. Nature established that during the transition to subsonic flow, due to singularity, the supersonic/hypersonic flows will create normal shock waves leading to the generation of entropy wave throughout the Sonic Jacket. The strength of the shock wave and/or the magnitude of entropy enhancement depends on the incoming flow Mach number and the heat capacity ratio of the gas. Across a shock wave, the static pressure, temperature, and gas density increases almost instantaneously. The changes in the flow properties are irreversible and the entropy of the entire system increases. Admittedly, to remove the singularity and achieve a smooth transition from hypersonic/supersonic flow to subsonic flow is a challenging task. Nevertheless, as envisaged by E.W.Beans (JPP 1994) for an internal flow system, the smooth transition from hypersonic to subsonic regime requires a unique relationship between area change, heat transfer, and frictional effects. This physical situation is similar to the benchmark condition set in a circular duct and/or a Streamtube for achieving the Sanal flow choking for diabatic flows. In this pilot paper, 2D and 3D in silico simulations have been carried out using various validated flow solvers to compare qualitatively the Standoff Distance of Sonic Ring of hypersonic and supersonic flying objects having classical shapes to demonstrate the concept. In silico results reveal that the Sonic Ring observed for sphere, bullet and a double wedge airfoil at hypersonic speeds are much closer to the surface as compared to the supersonic flow. This observation is a very significant contribution to any highspeed vehicle (M>1) design as it establishes the theoretical concept of Sonic Ring/Sonic Jacket and it further aids for the envelope design optimization of supersonic and hypersonic vehicles with confidence. We concluded that in addition to the geometry optimization, the Standoff Distance of Sonic Ring/Jacket can be increased by injecting fluid with a high heat-capacity-ratio than the operating fluid to the Streamtube flow choking region; and thereby we can reduce the intensity of aerodynamic heating by creating a thick shock layer region near to the hypersonic vehicles. The theoretical discovery of the standoff distance of Sonic Ring/Jacket presented herein is a paradigm shift in the design optimization of supersonic, hypersonic, and re-entry vehicles with credibility.